In the past, many conventional oil wells were operated by a downhole pump at or close to the bottom of the well, the pump being of a conventional reciprocating kind actuated by a rod string, in turn reciprocated vertically by a pump jack.
Many of these older reciprocating pumps have been recently replaced by rotary-drive progressive cavity pumps. Such rotary pumps are particularly suited for the production of crude oil laden with sand and water.
However, because of the typical depth of an oil well, the torque applied at the top of the rod string, and the resistance of the pump at the bottom, can cause the rod string to wind up like a spring, thus storing the torque energy. Whenever there is a power failure or the system is shut down, this stored torque energy, along with the energy created by the fluid head on the pump, must release itself. Without any control on the rate of backspin of the rod string, serious problems have occurred. The problems tend to be as follows:
the motor, connected to the rod string through a reducer and a sheave and pulley arrangement, may reach reverse speeds exceeding safe limits. These speeds tend to damage the motor, and can even cause it to explode. PA1 one or both of the sheaves can reach speeds exceeding their limits. PA1 on drive configurations in which the polish rod extrudes out the top of the drive, the projecting portion can bend and break, and the broken-off portion will then be flung away from the installation, due to centrifugal force. PA1 without some form of braking, the rod string could uncouple, with the result that the rod string and the pump would be lost down the hole. PA1 a) a rotary member mounted so that it rotates at a consistent speed ratio and direction with respect to the top end of the rod string, PA1 b) a fluid pump, PA1 c) a reservoir containing a fluid, PA1 d) an input conduit communicating the fluid in the reservoir with the intake of said fluid pump, PA1 e) an output conduit communicating the fluid in the reservoir with the output of said fluid pump, PA1 f) an adjustable flow-control valve located in one of said conduits, and PA1 g) an over-running clutch operatively associated with the fluid pump such that, when the top end of the rod string rotates in the direction corresponding to normal operating of the downhole pump, no pumping work is done by the fluid pump, but when the top end of the rod string rotates in the direction opposite that corresponding to normal operation of the downhole pump, the fluid pump does the work of pumping the fluid out of and then back to said reservoir against a resistance determined by the setting of the valve, PA1 a downhole pump which includes a stator and a rotor, PA1 a rod string having a top end and a bottom end, the bottom end being connected to, supporting and rotating said rotor, PA1 a prime mover providing torque energy for rotating said top end, whereby twist energy is stored in the rod string during operation, and PA1 a braking mechanism for avoiding a too sudden release of said twist energy in the rod string on shut down or power failure, the mechanism including: